[patch v2] mm, page_alloc: make first_page visible before PageTail

[David Rientjes <rientjes@google.com>]

Commit bf6bddf1924e ("mm: introduce compaction and migration for ballooned
pages") introduces page_count(page) into memory compaction which
dereferences page->first_page if PageTail(page).

Introduce a store memory barrier to ensure page->first_page is properly
initialized so that code that does page_count(page) on pages off the lru
always have a valid p->first_page.

Reported-by: Holger Kiehl <Holger.Kiehl@dwd.de>
Signed-off-by: David Rientjes <rientjes@google.com>
---
 v2: with commentary, per checkpatch

 mm/page_alloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -369,9 +369,11 @@ void prep_compound_page(struct page *page, unsigned long order)
 	__SetPageHead(page);
 	for (i = 1; i < nr_pages; i++) {
 		struct page *p = page + i;
-		__SetPageTail(p);
 		set_page_count(p, 0);
 		p->first_page = page;
+		/* Make sure p->first_page is always valid for PageTail() */
+		smp_wmb();
+		__SetPageTail(p);
 	}
 }
 

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